1ST Snake: 2645.30, 644.20, 17.44 2ND Snake: 2414.92, 915.96, -33.07 (My coords were taken on top of where I started to attack these snakes.) These snakes travel a little faster then the large and normal king snakes. If you don't want to use the coords I have given just head north of the house with the smiths in it, go almost 3/4's of the map until the path starts to turn a little then head northwest, stand at the highest point of the first hill you get to and look southwest to northwest and you should see a Giant King Snake(once it spawns) traveling it's path. My snakes were about 20 mins apart. The second snake I saw was actually stopped. Not sure if it was getting ready to despawn or just change direction.

I can verify this is a good guide. The snake hangs out on the left side of the map. Just wonder up and down the left side on the hills and kill any king snake/large king snake. Eventually, the giant king snake will spawn. Spawned twice for me an an hour of search all up. Dropped the snake skin once.